Polymeric films and articles are extensively used in optical applications. One major problem with the use of such materials is reflective losses at the substrate/air interface, resulting in lower intensity of transmitted light. Issues of reflective losses across multiple interfaces can be addressed by adjusting the refractive indices of the films. One such example is cured film of urethane acrylate resin, which is widely used as protective coat in variety of applications involving display devices. Although, urethane protective coats have excellent transparency, hardness and scratch resistance, it is difficult to modify their refractive indices due to limited choices of building materials that are available for optical applications. An alternative means of modifying refractive index is to use small amounts of miscible additives, which do not alter other fundamental properties such as transparency, hardness and scratch resistance.
High refractive index values of metal compounds make them ideal candidates as additives to boost refractive indices of organic polymeric materials. For instance, Arpac et al. in U.S. Pat. No. 6,291,070 describe use of several nanoscale inorganic particles to create molded articles of varying refractive indices. Practical utility of inorganic particles in boosting refractive index is greatly restricted by the limited compatibility between such particles and organic polymeric matrices. Processes such as “micronization” can produce nanoparticles with relatively high dispersion to some extent but there is a practical limit to the size achievable economically by “micronization”. For applications where transparency is important, the particle size must be smaller than the wavelength of the light in order for the material to be transparent. Sol-gel or solution-colloidal phase reactions are alternative means of generating very fine particles of metal oxides, but the nature of the small particles often leads to their agglomeration, causing increased hazing and scattering of a transparent article over time.
Issues of agglomeration of fine particles can be addressed through chemical surface reactions. For instance, inorganic particles, described by Arpac et al. in U.S. Pat. No. 6,291,070, were surface-treated with hydrolysable silane containing at least one polymerizable and/or polycondensable group. Chisholm et al. in U.S. Pat. No. 6,844,950 also describe the use of nanoparticles of ethylenically unsaturated compounds of zirconium and titanium. Similarly, Arney et al. in U.S. Pat. No. 6,432,526 describes the use of metal oxides modified with dispersing aids for improved compatibilization with organic materials. The main difficulty with this approach is that the actual nanoparticle compositions are changed by attaching these modifying species to them. Moreover, the metal concentration in any subsequent formulations is decreased by the presence of these organic functional groups. Most critically, the issues of hazing and light scattering after the article has been exposed to prolonged storage are not completely solved due to the limited shelf life of surface modified metal particles. Designing metal-containing compositions with homogeneous dispersion in the final article or the polymerizable fluid and long shelf life stability, therefore, continues to be a challenge.
Use of discrete metal compounds as processing aids and curing agents in the processing of certain types of elastomers and some dental compositions is known. For instance, Nagel et al in U.S. Pat. No. 6,194,504 describe the use of metal salts of acrylic acid as processing aids to improve dispersion of such curing additives in butadiene, natural rubber and EPDM based elastomers. Fabian in U.S. Pat. No. 6,553,169 and Shustack et al. in U.S. Pat. No. 6,656,990 describes the use of less than 0.5 weight-percent of titanates and zirconates as energy curable coupling agents to improve adhesive properties and dispersion of pigments. Similarly, use of zirconium-based acrylate as coupling agent between amorphous calcium phosphate and polymeric matrices has been reported by Skrtic et al. [Biomaterials 24 (2003) 2443-2449].
None of the art reported above teaches how to create an optically clear film or article with excellent physical and mechanical properties, especially ones with high refractive index, and improved shelf life from compositions containing discrete metal-containing functional precursor units.